The present invention relates in general to methods of controlling stress in materials, and in particular to methods of controlling internal and interfacial stresses between adjacent layers in multilayer structures.
When two dissimilar materials are placed in intimate contact with one another, such contact may give rise to stresses, particularly at the interface between the materials. This is illustrated in FIG. 1 where a multilayer material structure 1 is shown having a substrate material 3 that is bonded or joined in contact with a second material 6 using a technique such as vacuum deposition, pulsed laser deposition (PLD), chemical vapor deposition (CVD), or other known technique. The intimate contact between the substrate and second material 3, 6 gives rise to internal stresses, schematically illustrated as a stressed region 4. The stressed region 4 may arise by virtue of the inherent equilibrium properties of the dissimilar materials. The stressed region 4 may also arise as a response to externally applied fields such as temperature and pressure. In either respect, the imbalance of internal compressive or tensile stresses may cause problems in the multilayer material structure 1 such as cracks, delamination, surface defects, voids or degraded electrical or optical qualities or performance.
In view of the above problems, attempts have been made to control or eliminate the stress between two dissimilar materials. However, the majority of these attempts utilize an inter-coating or a buffer layer to provide a smooth gradient that ameliorates stress build-up at the interface between the materials. The inter-coating or buffer layer typically attempts to produce an opposite stress to the stress expected due to bonding of the materials, thereby partially canceling the stress. One problem with using an extra inter-coating or a buffer layer is that it takes additional time, space and materials to deposit the extra coating or buffer layer, thereby reducing manufacturing efficiency. In addition, the extra coating or layer may negatively affect the performance of the device in which the materials are used. Still further, it may not be possible to exactly match the undesired stress fields so that the approach may be only partially successful.
One attempt to control the internal stress between two dissimilar materials without the need of an extra coating or layer is described in U.S. Pat. No. 6,134,049 to Spiller et al. Spiller teaches a method of adjusting multilayer film stress induced deformation of optics so that stresses between layers are canceled without depositing an extra coating or layer on or in the film. Essentially, first and second layers of an overall multilayer optical structure must be selected among materials that exhibit both the property of generally equal and opposite stresses and the property that the first and second portions have optical constants that allow good reflectivity at a design wavelength. Thus, the manner in which the layers can be configured is limited. The first and second portion materials must be selected to have opposite stresses in order to produce a net film stress of zero. As such, only a negative stress material can be placed over a positive stress material. Still further, the layers must have optical constants that allow satisfactory reflectivity at a design wavelength.
Another approach to controlling the internal stress between to two dissimilar materials is taught in U.S. Pat. No. 6,037,420 to Tochioka. Tochioka teaches reducing the stress of materials by chemically placing an additive in a matrix at the molecular level. One disadvantage of this method is that the structure must be chemically altered which may affect the properties of the material and may not be desirable.
Thus, there is a need in the art for a method that will control the stress between two dissimilar materials that eliminates the need for extra coatings or layers, and allows for versatility in the types of materials used and the configuration of the materials. In addition, there is a need in the art for a method of controlling the stress between two dissimilar materials that is economically efficient and does not degrade the performance of the materials.